The present invention relates to an illumination device, and more particularly to a laser illumination device based on electrically switchable Bragg gratings.
Miniature solid-state lasers are currently being considered for a range of display applications. The competitive advantage of lasers in display applications results from increased lifetime, lower cost, higher brightness and improved color gamut. As lasers are polarized, they are ideally suited to Liquid Crystal on Silicon (LCoS) or High Temperature Poly Silicon (HTPS) projectors. In contrast to incoherent sources, lasers do not result in light from unwanted polarization states being discarded.
Laser displays suffer from speckle, a sparkly or granular structure seen in uniformly illuminated rough surfaces. Speckle arises from the high spatial and temporal coherence of lasers. Speckle reduces image sharpness and is distracting to the viewer.
Several approaches for reducing speckle contrast have been proposed based on spatial and temporal decorrelation of speckle patterns. More precisely, speckle reduction is based on averaging multiple (M) sets of speckle patterns from a speckle surface resolution cell with the averaging taking place over the human eye integration time. The speckle resolution cell is essentially the smallest area of the image that the eye can resolve. Under optimal conditions speckle contrast is reduced from unity to the square root of M. The value of M should be as large as possible. However, the value of M is limited by the numerical aperture of the imaging optics. In other words the minimum cell size is approximately equal to the laser wavelength divided by the numerical aperture.
Speckle may be characterized by the parameter speckle contrast which is defined as the ratio of the standard deviation of the speckle intensity to the mean speckle intensity. Temporally varying the phase pattern faster than the eye temporal resolution destroys the light spatial coherence, thereby reducing the speckle contrast.
The basic statistical properties of speckle are discussed by J. W. Goodman in a first paper entitled “Some Fundamental Properties of Speckle” (J. Opt. Soc. Am. 66, pp. 1145-1149, 1976) and a second paper entitled “Statistical Properties of Laser Speckle Patterns” (Topics in Applied Physics volume 9, edited by J. C. Dainty, pp. 9-75, Springer-Verlag, Berlin Heidelberg, 1984).
There are two types of speckle: objective and subjective speckle. As noted in an article by D. Gabor in the IBM Journal of Research and Development, Volume 14, Number 5, Page 509 (1970) “Objective” speckle arises from the uneven illumination of an object with a multiplicity of waves that interfere at its surface. “Subjective” speckle arises at rough objects even if they are illuminated evenly by a single wave. In practical terms, objective speckle results from scattering in the illumination system while subjective speckle occurs at the projection screen. As its name implies objective speckle is not influenced by the viewer's perception of the displayed image. A photographic emulsion spread over the surface of the object would record all of the key characteristics of objective speckle. Even a perfect optical system cannot do better than to reproduce it exactly. Subjective speckle on the other hand arises by a diffraction effect at the receiving optics or, more exactly, by the limitation of the amount of light admitted into receiving optics (the eye, in the case of a display). The only remedy for subjective speckle is to widen the aperture of the receiving optics or to perform an equivalent optical process. This is due to fundamental information theory limitations and not any practical optical consideration.
The characteristics of objective and subjective speckle may be illustrated by considering a typical projection system. The illumination and beam shaping optics (for example components such as diffusers or fly's eye integrators) generates scattering that eventually creates a speckle pattern onto the microdisplay panel surface. The projection lens images this pattern onto the screen giving the objective speckle pattern. The screen takes the objective speckle pattern and scatters it into the viewing space. The human eye only collects a tiny portion of this light. Since the objective speckle acts like a coherent illumination field, the diffusion of the screen produces a new speckle pattern at the retina with a different speckle grain. This is the subjective speckle pattern. The subjective speckle pattern will be influenced by screen diffuser materials and lenticular structures and other features commonly used in screens. Since a well-designed projection lens usually collects most of the light transmitted through or reflected by the microdisplay panel, the objective speckle pattern generated is well reproduced at the screen, allowing for some modification due to optical aberrations. The cumulative speckle seen by the eye is the sum of the objective and subjective speckles.
Removing the objective speckle is relatively easy since the speckle pattern is well transferred from the illumination to the screen: any change in the illumination will be transferred to the screen. Traditionally, the simplest way has been to use a rotating diffuser that provides multiplicity of speckle patterns while maintaining a uniform a time-averaged intensity profile. This type of approach is often referred to as angle diversity. Note that, if the objective speckle is suppressed at the screen, it will be suppressed at every plane between the projection lens and the screen.
Suppression of subjective speckle is more difficult. Because of large disparity between the projection optics and eye optics numerical apertures (or F-numbers), the objective speckle grain is much larger than the subjective speckle grain. Therefore, the objective speckle provides a relatively uniform illumination to the screen within one resolution cell of the eye regardless of the position of the rotating diffuser or other speckle reduction means in the illumination path. For the purposes of quantifying the subjective speckle it is convenient to define the speckle contrast as the ratio of the resolution spots of the eye and the projection optic at the screen.
The characteristics of speckle depend on whether it is observed in the near or far field. The far field of an optical system is the angular spectrum of the plane waves traversing or generated by the optical system. In case of a diffractive optical element such as a Computer Generated Hologram (CGH), the far field is a series of points located in the two dimensional angular spectrum, each point representing the intensity of a specific plane wave diffracted, refracted, reflected or diffused at a specific angle. If only one beam strikes the optical element, no overlap of plane waves occurs, each plane wave being spatially demultiplexed in the far field. This is not the case for the near field. The far field effectively at infinity, which according to Rayleigh-Sommerfeld theory is any distance after a specific finite distance, which is a function of the size of the beam (that is, the effective aperture of the CGH), the wavelength, the size of the microstructures in the element (amount of beam deflection), and other factors. Therefore, in order to change the speckle pattern of an individual beamlet in the far field, it is best to use phase diversity. Angular diversity would not produce good results, since none of the wave fronts would be overlapping and interfering. However, phase diversity would create a different phase pattern on a single beamlet and this would change the speckle. Speckle patterns in the far field are characterized by very small-grained speckle structures.
In the near field (that is any location closer than the Rayleigh-Sommerfeld distance), many different wave fronts are interfere and overlap resulting in a very large amount of local wave front interference and hence speckle. Therefore, in order to reduce speckle in the near field, it is advantageous to make slight variations to the angles of the overlapping beamlets. In other words, angular diversity despeckling schemes will be the most effective. Speckle in the near field is characterized by larger grains. The different grain structure in the near and far fields can lead to the erroneous conclusion that Fresnel CGH (near field) gives less speckle than Fourier CGHs (far field). This is not the case; the nature of the speckle is different in the two cases.
The extent to which speckle can be corrected in the near and far fields has implications for the type of despecklers to be used in specific projector applications. In the case of a laser projector using traditional projection imaging apparatus, the image of a microdisplay is not in the far field of the despeckler, and thus angular diversity would be the most effective solution. In the case of a laser projector using diffractive imaging, the image is actually the far field of the microdisplay itself, and very close to the far field of the despeckler. Therefore, it is best to use a combination of angular diversity and phase diversity.
Techniques for speckle reduction are commonly classified into the categories of angular, phase and wavelength diversity according to the optical property used to generate the speckle patterns. Angular diversity typically relies on the use of rotating diffusers or vibrating screens. Phase diversity is typically provided by electrically controlled phase modulators. Wavelength diversity is provided by multiple laser sources or tuneable single laser sources. In the case of laser arrays, speckle reduces as the inverse of the square root of the number of die. Mechanical methods of suppressing speckle suffer from the problems of noise, mechanical complexity and size.
It is known that speckle may be reduce by using an electro optic device to generate variation in the refractive index profile of material such that the phase fronts of light incident on the device are modulated in phase and or amplitude. The published Internal Patent Application No. WO/2007/015141 entitled LASER ILLUMINATOR discloses a despeckler based on a new type of electro optical device known as an Electrically Switchable Bragg Grating (ESBG).
An ESBG in its most basic form is formed by recording a volume phase grating, or hologram, in a polymer dispersed liquid crystal (PDLC) mixture. Typically, ESBG despeckler devices are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. Techniques for making and filling glass cells are well known in the liquid crystal display industry. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the PDLC layer. A volume phase grating is then recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the HPDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the PDLC layer. When an electric field is applied to the hologram via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range from near 100% efficiency with no voltage applied to essentially zero efficiency with a sufficiently high voltage applied. U.S. Pat. Nos. 5,942,157 and 5,751,452 describe monomer and liquid crystal material combinations suitable for fabricating ESBG despeckler devices. A publication by Butler et al. (“Diffractive properties of highly birefringent volume gratings: investigation”, Journal of the Optical Society of America B, Volume 19 No. 2, February 2002) describes analytical methods useful to design ESBG despeckler devices and provides numerous references to prior publications describing the fabrication and application of ESBG despeckler devices.
The apparatus disclosed in Internal Patent Application No. WO/2007/015141 suffers from the problem that insufficient speckle states are produced using the ESBG configurations taught therein.
It is a first object of the present invention to provide an ESBG despeckler device that can overcome the problem of laser speckle.
It is a second object of the present invention to provide a compact, efficient laser display incorporating an ESBG despeckler device that can overcome the problem of laser speckle.